Conventionally, a scanner unit as a document reading apparatus employs a halogen lamp, fluorescent lamp, or the like as a document illumination device. Recently, a xenon lamp is being used as a new illumination device. The xenon lamp, however, decreases in light quantity with the lapse of the time after the lamp is turned on. In addition, a means for adjusting the xenon lamp has not been developed yet. If document sheets are successively read, the image reading level varies.
In the use of the xenon lamp, therefore, this problem is solved by generating shading correction data at the start of image reading processing when, e.g., the start key of the scanner unit is pressed, and updating the shading correction data every predetermined time or every time a predetermined number of document sheets are read.
In addition, as disclosed in Japanese Patent Laid-Open No. 5-14712, a side white reference tape is pasted to a scan unit, and shading correction is done based on white reference information obtained by reading the side white reference tape. As disclosed in Japanese Patent Laid-Open No. 11-155048, the optical characteristics of a contact glass are arbitrarily controlled.
However, updating shading correction data during image reading decreases the image reading performance. Further, when the side white reference tape is pasted to the scan unit, proper correction cannot be achieved unless a decrease in the light quantity of the lamp is equal between the pasting position of the side white reference tape and the entire image reading area.